Honorific. Even more respectful than -san; the effect in modern-day Japanese conversation would be along the lines of "Milord So-and-So." As used by Kenshin, it indicates both respect and humility.

honorific, very polite; this was used in ancient times when referring to a lord or master

honorific used to show respect but not necessarily love

in historical or fantasy works, an honorific for somebody you respect; can be used for feudal lords

suffix added as an honorific when referring to one's own lord

Dono (Kanji: 殿) is an honorific in the Japanese language, rarely heard in current conversation. It is more common to early-period films or anime. Modern usage is limited to formal writings.